It is known to provide a tensioning nut with a sealing washer which outwardly locks the front side of the tensioning nut (i.e. the side facing away from the machine) for the purpose of providing a liquid tight seal. Conventional cylindrical surfaces of the opening facing towards the tool shank, include sealing washers having an annular groove in which an O-ring is disposed. The dimensions of the O-ring are such that it sits elastically against the tool shank thus closing the annular gap between the shank and the sealing washer. The seal is substantially provided by the elasticity of the O-ring.
When coolant internal pressures become elevated, the seal is not always assured. This is especially true when the elasticity and the surface of the O-ring have been degraded by prolonged use.